Long Polling example

main.dart

/*
 * Long Polling example
 * https://gist.github.com/PlugFox/51038cb6f587332f6e803790a7d991e5
 * https://dartpad.dev?id=51038cb6f587332f6e803790a7d991e5
 * Matiunin Mikhail <plugfox@gmail.com>, 29 October 2023
 */

// ignore_for_file: avoid_print

import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'dart:math' as math;

void main([List<String>? arguments]) {
  final fn = polling();
  Future.doWhile(() async {
    try {
      final messages = await fn();
      messages.forEach(print);
    } on Object {/* ignore */}
    return true;
  });
}

Future<List<({int id, String text})>> Function() polling([
  Duration timeout = const Duration(seconds: 12),
]) {
  const kEmptyList = <({int id, String text})>[];
  final url = Uri.parse('http://localhost:8080/getUpdates');
  final client = HttpClient();
  int? $lastId;
  Uri createUrl() => url.replace(queryParameters: <String, Object?>{
        if ($lastId != null) 'id': $lastId?.toString(),
        'timeout': timeout.inMilliseconds.toString(),
      });

  return () async {
    final request = await client.getUrl(createUrl());
    final response = await request.close();
    if (response.statusCode != HttpStatus.ok) throw Exception('bad_status_code');
    final body = await response.transform<String>(utf8.decoder).join();
    if (body.length < 3) return kEmptyList;
    final messages = (jsonDecode(body) as Iterable)
        .whereType<Map<String, Object?>>()
        .expand<({int id, String text})>((e) => switch (e) {
              {'id': int id, 'text': String text} => [(id: id, text: text)],
              _ => kEmptyList,
            })
        .toList(growable: false);
    if (messages.isEmpty) return kEmptyList;
    $lastId = <int>[
      if ($lastId != null) $lastId!,
      ...messages.map<int>((e) => e.id),
    ].reduce(math.max);
    return messages;
  };
}